However, what if the instanceA object does not exists yet, but I know it will exists when I call the corresponding function? MethodX and MethodY are on an external library that I am not supposed to modify.

-why: think about this situation: the class A has 100 Methods, each returning a different float depending on the class A state. However, depending on some other state, we may want to access only the first 5 methods, or only the first and the fourth method. The class state to which this method applies may change over time.
My idea was to have a big lists with all the 100 methods, then by using indexes corresponding to the method, create a sublist LL with only the appropriate methods (for example, [1,2,3,4,5], or [1,4]).
Then, once that the object A is created, I would run in turn all the different method in the sublist LL, somehow as they were called by the object A.